Constraints on cortical and thalamic projections: the no-strong-loops hypothesis

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The many distinct cortical areas of the macaque monkey visual system can be arranged hierarchically, but not in a unique way. We suggest that the connections between these cortical areas never form strong, directed loops. For connections between the visual cortex and particular thalamic nuclei, we predict that certain types of connections will not be found. If strong, directed loops were to exist, we suggest that the cortex would go into uncontrolled oscillations.

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Figure 1: Rules used by Felleman and Van Essen1 for deciding whether a projection between two co.
Figure 2: Forbidden cortical loops.
Figure 3: An acyclic digraph with eight nodes (A to H), drawn so that all the arrows point upwards.
Figure 4: Termination of a typical R (round)-type corticopulvinar axon and an E (extended) corticopulvinar axonal termination.
Figure 5: Permissible and forbidden connections between thalamus and cortex.
Figure 6: A hypothetical set of connections lacking strong directed loops.


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F.C. is supported by the J. W. Kieckhefer Foundation. C.K. is supported by the National Institute of Mental Health, the Office of Naval Research and the National Science Foundation. Especial thanks to Jan Kaas and Kathy Rockland.

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Correspondence to Francis Crick.

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